Add original revision extraction for signed PDFs

EnvelopeGenerator.Application/Common/Interfaces/Services/IPdfDigitalSignatureService.cs

@@ -35,4 +35,13 @@ public interface IPdfDigitalSignatureService
     /// <param name="cancellationToken">Cancellation token for cooperative cancellation.</param>
     /// <returns>A structured result containing signature presence and per-signature details.</returns>
     Task<PdfSignatureCheckResultDto> CheckPdfSignatureAsync(byte[] pdfBytes, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Extracts the original signed revision from a signed PDF.
+    /// For multi-revision files this returns the first signed revision bytes.
+    /// </summary>
+    /// <param name="pdfBytes">Signed PDF bytes.</param>
+    /// <param name="cancellationToken">Cancellation token for cooperative cancellation.</param>
+    /// <returns>PDF bytes of the original signed revision.</returns>
+    Task<byte[]> ExtractOriginalPdfAsync(byte[] pdfBytes, CancellationToken cancellationToken = default);
 }